U.S. patent number 4,869,112 [Application Number 07/268,464] was granted by the patent office on 1989-09-26 for screw-driven actuator for test frame.
This patent grant is currently assigned to MTS Systems Corporation. Invention is credited to Richard E. Bearden, Martin M. Gram.
United States Patent |
4,869,112 |
Gram , et al. |
September 26, 1989 |
**Please see images for:
( Certificate of Correction ) ** |
Screw-driven actuator for test frame
Abstract
A mechanical screw drive loading test frame has a crosshead on
the frame with a grip supported from the crosshead, and a second
grip for gripping a second end of a specimen on which a second grip
is connected to an axially movable shaft which is ragidly supported
and guided for axial movement for loading the specimen. The support
for the second grip is in a second rigid cross member positioned
very close to the second grip to prevent lateral loads from loading
the specimen transversly. The loading shaft is loaded from below
the rigid cross member by a screw through a coupling and mounting
arrangement which isolates load caused by misalignment in the drive
screw or drive motor from the movable shaft and second grip.
Inventors: |
Gram; Martin M. (St. Louis
Park, MN), Bearden; Richard E. (Arden Hills, MN) |
Assignee: |
MTS Systems Corporation (Eden
Prairie, MN)
|
Family
ID: |
23023115 |
Appl.
No.: |
07/268,464 |
Filed: |
November 8, 1988 |
Current U.S.
Class: |
73/856;
73/796 |
Current CPC
Class: |
G01N
3/08 (20130101) |
Current International
Class: |
G01N
3/08 (20060101); G01N 003/08 () |
Field of
Search: |
;73/856,796,826,827,828,829,830,831,832,833,834 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Instron Corporation, Catalog page. .
Schenk, Catalog page..
|
Primary Examiner: Myracle; Jerry W.
Attorney, Agent or Firm: Kinney & Lange
Claims
What is claimed is:
1. In a screw actuated test assembly, a test frame including a base
plate member for supporting specimen loading means, said specimen
loading means including grip means for attaching to an end of a
specimen and applying a load thereto, a shaft slidably mounted for
movement relative to said base plate, in directions along the
loading axis, said shaft being guided by bearing means through the
base member and being restrained from rotation, screw means
comprising an outer rotatable nut and an inner screw member
extending through the nut, one end of said inner screw member being
coupled to said shaft, drive means for said nut comprising a motor
coupled to drive said nut, said motor having an outer housing, and
flexure means connected between said motor housing and said test
frame to restrain the motor housing from rotation and to permit
said motor to deflect axially while being restrained from rotation
during the driving of said nut.
2. The apparatus as specified in claim 1 wherein the base member
comprises a cross member supported above a support surface and a
support housing on an opposite side of the base member from the end
of the shaft coupled to the grip, an end portion of said housing
opposite from said base member having bearing means supported
thereon, said nut being mounted in said bearing means for rotation
about the same axis as the axis of movement of said shaft.
3. The apparatus as specified in claim 2 wherein said flexure means
for supporting said motor is mounted to said support housing at the
lower end thereof.
4. The apparatus as specified in claim 2 wherein said bearing means
comprise a pair of bearings that securely restrain the nut from
axial or lateral movement, and wherein said coupling between said
shaft and said screw member is located between the bearing means
and the base member.
5. The apparatus as specified in claim 1 wherein said motor has an
internal central passageway, and said inner screw member being of
size to move axially along the internal central passageway as the
screw member is moved by rotation of the nut.
6. The apparatus as specified in claim 1 wherein said means for
guiding said shaft as it moves axially comprises a linear ball
bearing including a housing mounted on said base member, said shaft
having radially extending ribs thereon with balls on opposite sides
of said ribs, and said housing retaining said balls for rolling
support of said shaft in direction along its axis while restraining
rotation thereof.
7. A screw actuator for a test frame having an upper crosshead and
a lower base cross member, and grip means on the upper cross member
for supporting one end of a specimen to be tested, comprising a
shaft slidably mounted for movement relative to said base cross
member in direction along a loading axis, coinciding with a
longitudinal axis of the shaft, means for guiding said shaft on
said base cross member, means for mounting a second grip on a first
end of the shaft on a side of the base cross member facing toward
the upper cross member, a nut rotatably mounted relative to the
base cross member on a side of the base cross member opposite from
the means for mounting a second grip, a screw member threadably
extending through the nut, one end of said screw member being
coupled to an end of said shaft opposite from the first end, and
drive means for driving said nut comprising a motor, having an
output member coupled to the nut, said motor having an outer
housing mounted with respect to the base cross member through a
flexure plate which permits said motor to deflect on the
longitudinal axis while being restrained from rotation during the
driving of said nut.
8. The actuator as specified in claim 7 and a rigid housing mounted
to the base cross member on the same side thereof as the nut, said
housing forming a wall circumscribing said shaft, and an end
portion of said housing opposite from said base cross member having
bearing means thereon, said nut being mounted in said bearing means
and positioned between the motor and the base cross member.
9. The apparatus as specified in claim 8 wherein said flexure plate
has a center portion fixed to said motor and end portions
positioned laterally therefrom, the end portions being connected to
the center portion by thin flexure plate portions, the end portions
being supported relative to the housing.
10. The apparatus as specified in claim 8 wherein said bearing
means comprise a pair of bearings spaced in axial direction of the
shaft that securely restrain the nut from axial and lateral motion,
and a rigid coupling between said shaft and said screw member
located between the bearing means and the base cross member.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates test specimen loading machines that
substantially reduce unwanted lateral loads being applied to the
specimen while using servo-mechanical loading drives.
2. Description of the Prior Art.
Various screw-driven actuator tension compression test devices have
been advanced in the prior art, such as an electro-mechanical
actuator for a test frame made by the Instron Corporation, which
has a motor mounted below a test frame cross member, and a
non-rotating ball screw driven by a rotating nut. A high speed
motor is used to drive a gear box through a belt drive and the
output of the gear box rotates the nut to load a specimen coupled
to the ball screw through a specimen grip. The mounting for the
motor requires the use of a torque arm for preventing the motor
from rotating when the motor is driving the nut and screw.
Additionally, the Schenck Corporation sells a test frame which
utilizes a screw driven actuator with a motor that is mounted below
a main base on the test frame, and which uses a frame that is
guided on the columns for the test machine to attempt to eliminate
side loading of the specimen.
The above described devices require substantial reductions in motor
speed using transmissions, and the Schenck device does not provide
internal support for the movable loading shaft so that external
guiding is required or else side loads on the specimen are
present.
A number of patents have also issued for screw actuated test
machines, and in particular there are a number of patents which
show screws which move and load the main outer columns of test
machines. A typical device is shown in U.S. Pat. No. 3,859,848.
This test frame utilizes ball screw members for moving a crosshead.
The ball screw members form the side support columns.
A ball bearing vertical shaft guide is shown in U.S. Pat. No.
3,357,755, which provides for vertical movement of a shaft as
guided by a plurality of balls in an outer housing or race. This
guide is for a die-set, and provides a stablized axial guide.
U.S. Pat. No. 3,375,709 shows a loading device which has screw
actuated crossheads with edge guides.
U.S. Pat. No. 3,859,848 shows screw actuated crossheads as well
with independent supports shown for the loading device for the
grips.
U.S. Pat. No. 4,096,741 shows a central load member which is run
with a linear motor rather than a screw actuator. A screw is used
for taking up slack and providing stop members for a slider.
A sliding spline assembly that slides in a ball bearing housing,
which is of the type used for guiding a movable shaft in the
present device, is shown in U.S. Pat. No. 4,620,351. The ball-shaft
slide assembly that is used for stablizing the loading shaft in the
present test machine arrangement is commercially available.
A similar device that provides for nonrotation guiding of a shaft
inside a ball housing assembly is shown in U.S. Pat. No.
4,705,491.
U.S. Pat. No. 3,203,232 shows a testing machine with screws at the
outer edges of the crosshead used to move the crosshead. This type
of mechanical loading device shows ball screws for driving the
loading members.
SUMMARY OF THE INVENTION
The present invention relates to a screw-driven actuator for
testing specimens in a load frame and which is useful with
specimens which are quite brittle, such as ceramic specimens.
Lateral loads which cause a ceramic specimen to fracture quite
easily are substantially reduced or eliminated.
A specimen is loaded by a pair of grips holding opposite ends of
the specimen. One grip is supported from a crosshead mounted on
columns relative to a base, and the other grip is moved axially for
loading the specimen. The axial movement is generated by a
screw-drive assembly mounted below a rigid base cross member. The
screw-drive assembly in turn is coupled to provide axial movement
to a precisely guided nonrotatable loading shaft which directly
supports the second grip. The loading shaft is supported very close
to the second grip by a pre-loaded linear bearing that also
prevents shaft rotation so that there is little, or no bending of
the shaft caused by side loads. The loading shaft is thus precisely
axially centered with respect to the upper grip as well.
The loading assembly comprises a drive motor directly coupled to a
roller nut driving a roller screw. The screw is coupled to the
sliding loading shaft with a rigid coupling. The drive motor outer
housing is mounted to the test frame through a flexure plate so
that any deflection between the drive motor and the support for the
roller nut is accomodated by flexing the mounting plate. The
reaction loads on the motor from driving the screw are carried by
the flexure plate back to the rigid base member so that no torque
arms are used. The motor itself is a low speed (about 66 rpm) motor
that is driven without having the need for a gear reduction
system.
The roller nut is supported on bearings which tend to isolate any
loads from the motor with respect to the axially moving load screw
so that all of the parts are very precisely held.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view of a typical test machine utilizing
the loading arrangement of the present invention, with parts in
section and parts broken away;
FIG. 2 is a schematic cross section of a guide shaft used for
loading the specimen of the present invention; and
FIG. 3 is a side view of the test machine of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A specimen testing machine that uses mechanical screw loading for
the specimen is shown generally at 10, and includes upright columns
11, (four columns can be used if desired) which extend to support a
crosshead 12, which can be fixed in position at a desired vertical
location along the columns 11. The crosshead 12 has a grip 13
supported thereon with suitable support comprising a load cell 13A.
The grip 13 is a conventional design specimen testing grip that
holds an upper end of a brittle specimen 14. The lower end of the
specimen 14 is held in a second grip 15, which comprises a lower
grip and which is coupled to a vertically sliding, nonrotatable
load shaft 16. The load shaft has a number of splines or ribs 16A
that extend radially out from the main surface of the shaft. Each
of these splines or ribs 16A is precisely held between a series of
ball guide members 17, on opposite sides of each individual spline
16A. The ball members 17 are held in an outer race 18 that retains
the ball members precisely in position. The outer race 18 is fixed
to a very rigid base cross member 20 that forms part of the base 26
of the test frame 10. The rigid base cross member 20 provides a
very rigid support for the outer race 18, and thus for the shaft
16, with respect to the columns 11.
A large tubular support housing 27 is attached to the lower side of
the base cross member 20, and provides a rigid support depending
from the base cross member. Support housing 27 forms a column
support that has an interior center opening or passageway 29, and
adjacent the lower end thereof there are a pair of vertically
spaced bearings 30 and 31 that mount a hub assembly 32 made up of
sections 32A and 32B, which in turn is fixed to a nut 33 of a
conventional roller screw. A roller screw is substantially similar
to what is known as a ball screw, and is a commercially available,
low friction fine thread screw. The nut 33 comprises two roller
screw nuts loaded back-to-back on a single roller screw member 34
to eliminate axial backlash and the nut is mounted to drive the
screw member axially when the nut is rotated. The screw member 34
has an upper end shown generally at 35 in dotted lines that is
clamped with a suitable rigid clamp or coupling 36 to an end
portion 37 of the load shaft 16. The coupling 36 can be a split
clamp used to tightly clamp the screw and shaft together. The shaft
16 cannot rotate so screw 34 does not rotate either.
The screw 34 can be used to move shaft 16 either upwardly or
downwardly from a center of travel position. The specimen loading
is in either a tension or compression from an initial position of
the coupling 36 such as that shown in dotted lines. The nut will
move the coupling toward or away from the hub portion 32A. The nut
33 is suitably driven with cap screws from hub portion 32B.
A bearing retainer 41 is connected to the lower end of column 47
and carries a wiper which keeps foreign material from the bearings
and the threads of the screw 34 during use.
The hub or housing portion 32B has an outer drive flange 42 mounted
on the lower end thereof, and drive flange 42 is in turn coupled to
the output drive plate 43 of a high torque, low speed motor 44. The
motor 44 has an interior through bore so that the end of the screw
34 will pass through the center of the motor, but is not directly
driven by the motor. The motor drive is through the member 43, the
flange 42 and the hub 32 to the nut 33. The hub portions 32A and
32B also form bearing sleeves that fit into bearings 30 and 31.
The outer housing of the motor 44 is restrained from rotating, to
react the torque applied to the nut 33, through the use of a
flexure coupling that permits the motor to deflect vertically when
axial loads cause deflection in between the inner and outer uses of
bearings 30 and 31. In this form of the invention, the mounting
column or sleeve 27 has a pair of side flanges 45, and the flanges
45 have support plates of suitable width and thickness indicated at
46 fixed thereto with suitable cap screws and extending downwardly
generally parallel to the axis of the shaft 16 and the screw 34.
Plates 46 are to the exterior of the housing for the motor 44, and
at the lower end of the plates, which is below the lower end of the
motor 44, there is a flexure plate 47 mounted to the support
plate.
The flexure plate 47 is a rectangular plate that is fastened to the
bottom ends of the support plates 46 with suitable cap screws 48,
and which has a center plate portion 50 that is fastened to the
housing 44A of the motor 44 with suitable cap screws 51. A rib 49
is on each end of the flexure plate for attachment to the
respective support plate 46. The ribs 49 are held with the cap
screws 48 and are joined to the center disc 50 through a pair of
thin flexure type webs 52 that are integral with the bosses 49 and
the center disc 50. The support plates 46, acting through the
flanges 45 and the flexure plate 47 restrain the motor from
rotating when it is driving the nut 33 and thus is moving the
roller screw in one of the directions indicated by the arrow 24.
The flexure webs 52 permit the motor 44 to deflect, while keeping
it restrained from rotation, so that loads that might be caused by
slight misalignments or deflections are not transmitted to the
loading shaft 16, and this insures that the loading shaft 16 will
be guided in proper position. The guide balls for the ribs or
splines 16A insure that there is no rotation or lateral movement of
the shaft 16 so that the specimen 14 is not torsionally or
laterally loaded.
An LVDT transducer 60 is supported on a cross channel 61 supported
below the support plates 46. The LVDT extends through a bore in the
roller screw 34 and has its movement sensing rod attached to the
shaft 16 with a pin at the end of the shaft 16 to sense axial
movement of the roller screw 34, loading shaft 16 and grip 15. This
provides a co-axial mounting for the LVDT to provide position
sensing that is less sensitive to perturbations, misalignments or
deflections of the screw column.
The loading shaft 16 and thus the grip 15 are supported in the
lateral direction very close to the base cross member 20. As can be
seen, the grip 15 can be threaded to the end of the loading shaft
16 in a conventional manner and tightened down securely so that the
end of the grip and the end of the shaft abut under load.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention.
* * * * *